The invention relates to motion pictures and more specifically to an analog-digitizer sound system for motion pictures.
The present standard for sound-tracks imaged on motion picture film dates back to 1967 wherein the location on, and the area covered by stereo sound tracks on film was specified. This standard describes the dimensions of the tracks and the related standards used today. This method places two tracks along one edge of the film which are of two types, variable density and variable area, the last of which is used almost exclusively. Inherent to this method of reproducing sound is the undesirable effect of background noise and rumble due to the nature of the plastic medium and residue of the film emulsion passing through the light beam of the photo sound detector. Dolby Corporation with its Dolby B,C,H, etc has pretty much dominated the industry both with its noise reduction systems for movie film and those for magnetic tape. However even with the best system Dolby can offer, the sound suffers distortions and amplitude variations caused by the extreme processing needed to remove random noise from between the normal sound peaks.
The introduction of the compact disk or CD with its almost perfect sound reproduction ability has stimulated interest by several companies to try and incorporate digital-quality sound on film.
This has proven to be a formidable problem because of the immense amount of digital information required to produce the multiple channel digital formats while keeping the old stereo sound tracks intact for general use by most movie theaters. Also the cost of the digital reader-heads to decode the new digital sound tracks is very high. Dolby Laboratories has quite recently developed its Theater Digital System that is currently being tested in a number of theaters. Sony Corporation is expected to introduce their digital system in the near future. It should be noted that these systems are expensive and complicated. Also the economics of necessary maintenance and the actual working-life of a digitally-encoded film sound-track has yet to be established.
None of the new digital sound systems that are being developed by these major corporations at great expense, does anything to improve the old analog film sound-track that must remain on the film.
The background of the present invention relates to the reproduction of sound from motion picture film. Two of the biggest hurdles to overcome in the effort to improve sound from movie film are the increase of high frequency response and the reduction of background noise. Filters to reduce background noise also reduce high frequency response which forces the user to use compression and dynamic filter techniques. These lead to unwanted distortions and complexity.
Present analog sound track readers used in moving picture theater projectors read the variable width sound tracks on motion picture film by back lighting the sound track portion of the film with a focused slit of light which is arranged perpendicular to the direction of travel of the film and just wide enough to span both tracks of the normal stereo print. A dual photo detector is placed on the opposite sides of the film to intercept the light which passes through the sound track portion of the film. The two tracks, which are transparent to light, vary in width as the film moves past the slit light source. The thickness of the slit light source and the instantaneous width of the sound track as it passes the photo detector determines the total amount of light falling on the photo detector and in turn the amount of electrical output from the detector. As the film continues to move past the detector the sound information which modulates the width of the tracks is converted to an electrical audio output signal. Present photodiode detectors are linear devices and any change in the amount of light falling on the detector causes a corresponding electrical output. Electrical output changes due to different changes in the width of the sound tracks cause useful output. However changes due to residual emulsion, scratches, dirt and light aberration through the film plastic medium cause unwanted light modulation and are perceived as background noises in the electrical output.
The frequency response of present systems, irrespective of the noise limitation, is ultimately limited by the recording camera response. In reality, the limitation is based on the thickness of the slit light source used during playback. The best of the slit lenses produce about a 0.5 mil slit thickness which produces a high frequency limit of about 18 KHZ. These wide band slit lenses are usually used in special playback systems such as Dolby stereo or Surround Sound and are followed by dynamic noise filters and expanders. These require that the film be specially recorded with compression techniques to improve signal to noise ratios. Generally slit sizes are used which produce high frequency limits of about 9 to 16 KHZ.
The prior art patents of Johnson and Paul show how major improvement in frequency response is achieved by scanning dark to light boundaries in the sound track area rather than detecting the total amount of light passing through it. They also show how the inherent analog noises which are caused by light aberration in the film medium film emulsion residue and irregular light transmission through the film are ignored. The scanning method used by Johnson #4,124,784 detects the first dark to light boundary of the sound track area and generates a pulse which has one edge that varies in time with respect to the first dark to light boundary of the sound track and a fixed edge which is established by the end of the CCD scanning device. This width modulated pulse is filtered and converted to an audio signal by a pulse width to voltage converter.
Paul U.S. Pat. No. 5,231,627 also uses a linear CCD array to detect both boundary transitions of each variable width sound track on film and processes the CCD data to produce a stream of width modulated pulses which are ultimately separated into separate channels and converted into two channels of audio.
The indicated methods and devices used by Johnson and Paul greatly increase the frequency response and effectively reduce the hiss and rumble caused by the film medium in variable width sound track play back systems. However, other types of noise exist in variable width sound track sound systems. The worst of which is noise due to scratches, blotches and missing areas of dark emulsion. The reproduction of this type of track noise manifests itself as pops, cracks and thumps. Compared to the low level rumble and hiss noise generated in the film medium which is a baseline noise. The pops, cracks and thumps are very noticeable and can be as large as full transition where audio output signals if not limited will go to full to peak amplitude. The elimination of pop-crack-thump noise has not been addressed by the above patented systems except in the Johnson system wherein the first dark-light boundary in the variable width sound track is detected and begins a timing cycle which ends when the CCD scanner reaches end of scan. During this time the output of the CCD scanner is ignored until end of scan thereby eliminating all noise after the detection on the first boundary transition. A corresponding pulse is generated which has a width proportional to the time from detection of the first light dark boundary to the end of scan. The result is all blotches and scratches are ignored after the first boundary is detected. This first method is good except for scratches and blotches which fall on the first boundary area.